1. Field of the Invention
The present invention relates to an optical head for optically reading and/or writing data from/on an optical data storage medium (such as an optical disc) including multiple data storage layers and also relates to a read/write drive including such an optical head.
2. Description of the Related Art
An optical disc is known as a typical optical data storage medium from/on which data is optically read/written. An optical disc drive including an optical head is used to read and/or write data from/on an optical disc. Specifically, in reading and writing data from/on a given optical disc, the optical disc drive gets the optical disc irradiated with a certain quantity of light by the optical head and then detects the light that has been reflected from the optical disc. In this case, the optical disc drive controls a position of a focal spot of the light beam not only perpendicularly to the optical disc (i.e., a focus control) but also in the radial direction of the optical disc (i.e., a tracking control). The focus control is needed to focus the light beam right on a data storage layer, while the tracking control is needed to make the spot of the light beam always follow the target track on the data storage layer.
Recently, a variety of optical discs on the market often include a number of data storage layers to increase the maximum storage capacity. To read and/or write data from/on such an optical disc, a specially designed optical head and an optical disc drive including such an optical head are reported in various documents. If a given optical disc includes a number of data storage layers in this manner, the optical disc drive has to determine whether or not the data storage layer on which the focal spot of the light beam is currently located is the target layer to read data from and/or write data on.
For example, the optical disc drive disclosed in Japanese Laid-Open Publication No. 9-259456 carries out a tracking control operation on an optical disc with two data storage layers by a three-beam technique that uses one main beam and two sub-beams on the right- and left-hand sides of the main beam. The optical disc drive also senses, by using these sub-beams, on which data storage layer the focal spot is currently located. Hereinafter, that focus finding operation performed by this optical disc drive will be described.
First, referring to FIG. 14A, illustrated is a state in which the focal spot of light 111 is located on the deeper one (L0) of the two data storage layers L0 and L1 of an optical disc 105. The light 111, which has been emitted from a light source included in the optical head (not shown) of an optical disc drive, passes through an objective lens 110 to be incident onto the optical disc 105. In the example illustrated in FIG. 14A, the two data storage layers are respectively identified by the reference numerals L1 and L0 in the order in which the light 111 passes and will be simply referred to herein as the “layer L1” and “layer L0”, respectively. After having been incident onto the layer L1, the light 111 is then reflected by the layer L0 and follows the same optical path in the opposite direction so as to be incident onto the objective lens 110 again and then detected at the light detector of the optical head.
FIG. 14B shows the light detector 112 of the optical head and the light that has been reflected from the layer L0 and then received at the light detector 112. This optical disc drive uses the three-beam technique. Accordingly, at this point in time, the reflected light 111 has already been split into a main beam and two sub-beams.
The light detector 112 includes a quadrant photodetector 112a for receiving the main beam, two photodetectors 112b and 112c for receiving sub-beams to detect a tracking error signal, and another two photodetectors 112d and 112e for receiving sub-beams to find on which layer the focal spot is now located. In the example illustrated in FIG. 14A, the light 111 is focused on the layer L0. Accordingly, the light that has been reflected from the layer L0 (i.e., a layer L0 main beam and two layer L0 sub-beams) is incident onto the photodetectors 112a, 112b and 112c to form beam spots that do not exceed the outer edges of the photodetectors 112a, 112b and 112c. On the other hand, the light that has been reflected from the layer L1 (i.e., a layer L1 main beam and two layer L1 sub-beams) have expanded beam shapes, and the gap between the two sub-beams broadens. In FIG. 14B, the beam shapes of the layer L1 main and sub-beams received are indicated by the dashed circles. As shown in FIG. 14B, the layer L1 main and sub-beams are all greater in cross-sectional area than the photodetectors 112a, 112b and 112c. Looking at the other two photodetectors 112d and 112e, it can be seen that the light that has been reflected from the layer L1 is received in the greater quantity by the photodetector 112e than by the photodetector 112d. 
On the other hand, FIG. 15A illustrates a state in which the focal spot of the light 111 that has passed through the objective lens 110 is located on the shallower one (L1) of the two data storage layers L0 and L1 of the optical disc 105. In this case, the light that has been reflected from the optical disc 105 is received as shown in FIG. 15B. FIG. 15B shows the light detector 112 of the optical head and the light that has been reflected from the layer L1 and then received at the light detector 112. The light detector 112 shown in FIG. 15B has the same arrangement as the counterpart shown in FIG. 14B. In the example illustrated in FIG. 15A, the light 111 is focused on the layer L1. Accordingly, the light that has been reflected from the layer L1 (i.e., a layer L1 main beam and two layer L1 sub-beams) is incident onto the photodetectors 112a, 112b and 112c to form beam spots that do not exceed the outer edges of the photodetectors 112a, 112b and 112c. On the other hand, the light that has been reflected from the layer L0 (i.e., a layer L0 main beam and two layer L0 sub-beams) has expanded beam shapes, and the gap between the two sub-beams narrows. In FIG. 15B, the beam shapes of the layer L1 main and sub-beams received are indicated by the dashed circles. As shown in FIG. 15B, the layer L0 main and sub-beams are all greater in cross-sectional area than the photodetectors 112a, 112b and 112c. Looking at the other two photodetectors 112d and 112e, it can be seen that the light that has been reflected from the layer L0 is received in the greater quantity by the photodetector 112d than by the photodetector 112e. 
Accordingly, by comparing the quantities of light received by the photodetectors 112d and 112e with each other, the optical disc drive can determine whether the focal spot is located on the layer L0 or on the layer L1. More specifically, if the quantity of light received by the photodetector 112d is greater than that of light received by the photodetector 112e, then it can be seen that the focal spot is currently located on the layer L1. On the other hand, if the quantity of light received by the photodetector 112e is greater than that of light received by the photodetector 112d, then it can be seen that the focal spot is currently located on the layer L0.
However, the conventional layer sensing method adopts the three-beam method that uses sub-beams, and therefore, is never applicable for use in any optical head that adopts a single-beam method using no sub-beams or a read/write drive including such an optical head. This is not advantageous because the three-beam method is much more complicated in required arrangement and processing than the single-beam method. Thus, there is a lot of demand for the technique of locating the focal spot accurately by the single-beam method that uses a simpler arrangement.